Before the electric flash, two types of flash cubes, each with four single-use flush bulbs, were widely used on flashes. The bulbs contain chemicals to burn and generate light. In these flashes, battery power is used to trigger the fire in the bulb.
There are two types of the flash cubes, X and N. Unfortunately, they are not compatible. Therefore, the question here is how to distinguish them. Look at the following image of them side by side:
On the bottom of X-type flash cube (left), the central hole is in a square shape and no metal wires can be seen. A sign of X is often (not always) marked on the top of X-type flash cube. On the base of N-type (right), the central hole is round and four pairs of wire as contacts (total 8) can been seen.